Nitride layers are used in several places in the fabrication of semiconductor chips. Nitride layers are used as a hard mask for conventional LOCOS field oxide formation. They are also used as a hard mask and oxide polish stop for shallow trench isolation (STI). In both LOCOS field oxide formation and shallow trench isolation, the nitride layer is deposited on a thin pad oxide layer (100 to 250 .ANG.), and the nitride must be removed after field oxide formation. Typically, the nitride is removed in a phosphoric acid (H.sub.3 PO.sub.4) bath. The phosphoric acid is mixed with water and held in a boiling state in a closed system, so that the vapor is recondensed into the bath. The temperature at which the phosphoric acid and water mixture boils is a function of the concentration of the phosphoric acid and atmospheric pressure. The temperature for the boiling point at 760 mm Hg of the phosphoric acid and water mixture ranges from 100.degree. C. (pure water) to approximately 250.degree. C. (pure phosphoric acid). The range of temperatures for a phosphoric acid bath is typically from 130.degree. C. to 170.degree. C.
"The Etching of Silicon Nitride in Phosphoric Acid with Silicon Dioxide as a Mask" by W. van Gelder and V. E. Hauser, J. Electrochem. Soc., Solid State Science. vol. 114, No. 8, pp.869-872 (1967) discloses that nitride, oxide, and silicon etch rates increase exponentially with the increase in temperature with different activation energies. Thus, nitride:oxide and nitride:silicon selectivities are also a function of temperature. At higher temperatures, although the etch rates of nitride, oxide, and silicon are higher, nitride:oxide and nitride:silicon selectivities are lower.
"Mechanistic Study of Silicon-Nitride-Etching with Hot Phosphoric Acid" by K. Sato (Tohoku University), F. W. Kern (Tohoku University), T. Ohmi (Tohoku University), Y. Yamazaki (Nihon Chemical Industrial Co., ltd.), G. Sato (Nihon Chemical Industrial Co. ltd.), T Kaji (Nisso Engineering Co., ltd.), and H. Horiki (Nisso Engineering Co., ltd) discloses that the solubility of silicon increases as the temperature of the phosphoric acid bath increases. The nitride etching process adds silicon to the phosphoric acid bath. As more silicon wafers are run through the phosphoric acid bath, the silicon concentration in the bath increases. Once the silicon concentration reaches the saturation point, silicon precipitates are formed, which can deposit as defects. The phosphoric acid bath should be changed before the silicon begins to precipitate.
In addition, in the formation of semiconductors which use a nitride mask, often the process produces a thin oxide layer over the nitride mask. Highly selective etching of the nitride layer may be slowed by the thin oxide layer.
It would be desirable to obtain faster etch rates with high nitride:oxide and nitride:silicon selectivities. It would also be desirable to minimize the changing of phosphoric acid baths without increasing silicon precipitates.